The paints and coatings industry is continually looking for new formulations which meet demands for better product performance. In addition, the industry is faced with increasing pressure from regulatory efforts to reduce the release of volatile organic solvents into the environment when coating formulations are applied to a substrate. This has led the industry to step up efforts to develop high performance, resin-based architectural coating formulations in which the resin is water-borne, i.e. dispersed or emulsified in water, thus reducing the amount of volatile organic solvents present.
It is well known that a suitable coupling solvent needs to be present in water-borne resin coating compositions to achieve acceptable film forming and other properties when applied to a substrate. The resins used in coating compositions are normally not dissolved by water. The coupling solvent increases the mutual solubility between the resin and the water thus increasing the homogeneity of a coating composition and improving its performance when applied as a protective coating. Normally, sufficient coupling solvent must be present to maintain a single phase resin/water mixture for any ratio of water to resin. Because compounds useful as coupling solvents are relatively expensive and in light of the effort to reduce the presence of volatile organic compounds in coating compositions, there is a need to find solvents having a high degree of coupling power or coupling efficiency so as to achieve the desired results with a minimum amount of solvent present. Coupling power efficiency may be quantified as a "coupling number", which number represents the minimum mole percent of solvent required to ensure a single phase system at any ratio of water to resin.
Glycol ethers have been recognized as useful coupling solvents. In an article titled "Fundamentals of Glycol Ethers in Coupling Solvents", D. K. Pollock and R. J. LaTulip, The Dow Chemical Company, presented at Proceedings of the ESD/ASM, Advanced Coatings Technology Conference, Jun. 10-12, 1991, Dearborn, Mich., the coupling efficiency of glycol ethers prepared from ethylene oxide and propylene oxide having an alkyl group of one to four carbon atoms were evaluated in various hydrocarbon/water mixtures. Ethylene glycol ethers, in particular ethylene glycol butyl ether, was found to perform satisfactorily as a coupling agent in mixtures of lower hydrocarbons such as cyclohexane or decane and water. However, all of the ethylene oxide based glycol ethers tested, including ethylene glycol butyl ether, were found to be too hydrophilic to perform satisfactorily as a coupling solvent in mixtures of water and higher hydrocarbons such as hexadecane.
U.S. Pat. No. 5,753,738 describes certain alkoxy ethoxyethanol compounds which are useful as coupling solvents. The coupling efficiency of these diethylene glycol ethers was tested in mixtures of heptane and water and in aqueous resin systems and compared to commercially available coupling solvents such as ethylene glycol butyl ether, ethylene glycol hexyl ether and diethylene glycol butyl ether. The data in tables 1 to 5 of this patent suggests that among the commercially available coupling solvents tested ethylene glycol butyl ether has the best coupling efficiency followed by diethylene glycol butyl ether with ethylene glycol hexyl ether being the least effective.